WO2017025862A1

WO2017025862A1 - Method for reducing color of lignosulfonates

Info

Publication number: WO2017025862A1
Application number: PCT/IB2016/054666
Authority: WO
Inventors: Tian Shi YE; Weiwei DING; Yu Mei
Original assignee: Basf Se; Basf (China) Company Limited
Priority date: 2015-08-10
Filing date: 2016-08-03
Publication date: 2017-02-16

Abstract

Disclosed is a method of reducing the color of lignosulfonates, which is carried out by adding a reducing agent and a chelating agent. The lighter color lignosulfonate solutions prepared by the present process exhibits very low staining, good heat stability, and without any additional harmful impact to the application.

Description

METHOD FOR REDUCING COLOR OF LIGNOSULFONATES

Technical Field The invention relates to reducing the color of lignosulfonates. Particularly, the invention relates to a method for reducing the color of lignosulfonates which is carried out by adding a reducing agent and a chelating agent.

Background Art

Lignin is a complex polymer of aromatic alcohols and most commonly derived from woody or other herbaceous plants. There are many different types of derivants based on lignin due to its diversified functionalities, and among which, the sulfonated lignin materials, the main by-product of paper pulping, have found widespread use as surfactants, coagulants, emulsifiers, resin ex- tenders, dye dispersants and in particular as leather retanning agents. However, their dark color still has excluded them from certain uses, such as textile dye dispersant and retanning agents of light color leather due to the staining or darkening effect.

Lignin itself in wood is almost colorless. Lignin isolation procedures and pulping procedures invariably introduce a variety of chromophores, for example chromophores with conjugated structure, such as quinonoids, into the structure and render the isolated lignins strongly colored.

The aforementioned conjugated systems may be saturated by reductive processes to achieve some reduction of lignin color. However, the reduction effect in the known reductive processes are not stable under the influence of oxygen (air) and sunlight.

Hosoya, Shyuji, et.al., "Brightening of Lignin in Pulp with Sodium Hydrosulfite", Mokuzai Gakkaishi (1970), 16(3), 140-4 CODEN: MKZGA7; I SSN:0021-7-4795, describes treating Ca lignosulfonate from softwood with an aqueous solution (Ph 6.5) of Na2S20₄ at 70°C for 5-180 minutes. In this process, lignosulfonate was treated by a large amount (one to thirty times) of Na2S204 for color reduction, which is not economically feasible. The process also required elevated temperature, which increases the risk of color reversion.

On the other hand, the aforementioned conjugated systems may be cleaved by oxidative pro- cesses to reduce the color of lignin. Actually, the research works in recent years have been focused on oxidative processes to achieve color reduction by cleaving the conjugated systems, because the colorless end-products in oxidation reactions are stable and chromophores are not reformed thereof. For example, US 4,184,845 discloses a two-step process comprising a first step of blocking the phenolic hydroxyl group and then oxidizing the blocked lignin, whereby the color of lignosulfonates are drastically reduced. However, such oxidation process is a complicated treatment process involving elevated pressure of oxidation gas and elevated temperature control which limit the application of such method in the color reduction. Summary of the Invention

The purpose of the present invention is to provide a simple method whereby the color of ligno- sulfonates is dramatically reduced, and the reduction of the color is stable enough to industry requirement.

The foregoing purpose can be realized by the present method, which comprises the following aspects: 1. A method for reducing the color of lignosulfonates, comprising mixing a reducing agent and a chelating agent with the lignosulfonates.

2. The method for reducing the color of lignosulfonates according to the above 1 , which is carried out by adding a reducing agent followed by adding a chelating agent into the lig- nosulfonates.

3. The method for reducing the color of lignosulfonates according to the above 1 or 2, which is carried out at room temperature and atmospheric pressure. 4. The method for reducing the color of lignosulfonates according to any of the above 1-3, wherein the chelating agent is an EDTA salt.

The method for reducing the color of lignosulfonates according to the above 4, wherein the EDTA salt comprises an alkali metal and/or an alkaline earth metal salt of EDTA.

The method for reducing the color of lignosulfonates according to the above 5, wherein the EDTA salt comprises Mg, K, 2K, 2Na, and/or 4Na salts of EDTA.

The method for reducing the color of lignosulfonates according to any of the above 1-6, wherein the amount of the chelating agent is at least 0.5% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to the above 7, wherein the amount of the chelating agent is from 0.5% by weight to 10% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to the above 8, wherein the amount of the chelating agent is from 0.5% by weight to 5% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to the above 9, wherein the amount of the chelating agent is from 0.5% by weight to 2% by weight, with respect to the total solids of the lignosulfonates. 1 1. The method for reducing the color of lignosulfonates according to the above 10, wherein the amount of the chelating agent is from 0.5% by weight to 1 % by weight, with respect to the total solids of the lignosulfonates. 12. The method for reducing the color of lignosulfonates according to any of the above 1-1 1 , wherein the reducing agent is a hydrosulfite.

The method for reducing the color of lignosulfonates according to the above 12, wherein the reducing agent is sodium hydrosulfite.

The method for reducing the color of lignosulfonates according to any of the above 1-13, wherein the amount of the reducing agent is above 0.5% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to the above 14, wherein the amount of the reducing agent is from 0.5% by weight to 10% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to the above 15, wherein the amount of the reducing agent is from 0.5% by weight to 5% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to the above 16, wherein the amount of the reducing agent is from 0.5% by weight to 3% by weight, with respect to the total solids of the lignosulfonates.

18. The method for reducing the color of lignosulfonates according to the above 17, wherein the amount of the reducing agent is from 0.5% by weight to 2% by weight, with respect to the total solids of the lignosulfonates.

A simple method to reduce the color of lignosulfonates is disclosed here. The color reduction effect is achieved and retained for an ideal time period by the use of a reducing agent in combination with a chelating agent. The method can be easily embedded into the current normal lignosulfonates production process without significant impact on the production cost.

The present method is a simple method without the need of using elevated temperature and elevated pressure, using less materials, with good stability, lower cost, and eco-friendly.

Brief Description of the Drawings

Figure 1 shows the appearances of calcium lignosulfonate aqueous solutions with a concentra tion of 1 % by weight at different pH. Figure 2 shows the appearances of calcium lignosulfonate aqueous solutions in which 0wt%, 1wt% and 3wt% of sodium hydrosulfite, with respect to the total solids of the lignosulfonate, was added respectively. The concentration of calcium lignosulfonate was 1 % by weight, and pH of the solutions was 6.

Figure 3 shows the appearances of calcium lignosulfonate solutions without EDTA-4Na (tetra- sodium Ethylene Diamine Tetraacetate) (a) and with EDTA-4Na (b). The concentration of calcium lignosulfonate was 1 % by weight, pH of the solution was 6. Sodium hyposulfite content was 1 % with respect to the total solids of the calcium lignosulfonate, EDTA-4Na content was 1 % with respect to the total solids of the calcium lignosulfonate.

Figure 4 shows the appearances of the calcium lignosulfonates solutions according to test 1 .

Figure 5 shows the appearances of the calcium lignosulfonates solutions according to test 2.

Figure 6 shows the appearances of the calcium lignosulfonates solutions according to test 3. Figure 7 shows the appearances of the calcium lignosulfonates solutions according to test 4. Figure 8 shows the appearances of the calcium lignosulfonates solutions according to test 5. Figure 9 shows the appearances of the calcium lignosulfonates solutions according to test 6. Mode for Carrying Out the Invention

The present invention relates to a method for reducing the color of lignosulfonates using a reducing agent and a chelating agent.

The present invention surprisingly found that the combination of a reducing agent and a chelat- ing agent could make the color reduction of lignosulfonates stable.

In the present invention, any reducing agent known in the art could be used, for example, a bisulfite, a hydrosulfite, and/or sulfurous acid, preferably a hydrosulfite, more preferably sodium hydrosulfite.

The amount of the reducing agent is above 0.5% by weight, preferably above 0.8% by weight, with respect to the total solids of the lignosulfonates. Generally, 0.5% by weight of the reducing agent is enough to achieve a desired bleaching effect, while the amount of the reducing agent can be increased according to the practical need. And, generally, increasing the amount of the reducing agent would enhance the bleaching effect. According to the invention, the amount of the reducing agent may be no more than 5% by weight. Preferably, the amount of the reducing agent is below 2% by weight. The present invention uses less materials, which provides lower cost, and also eco-friendly. The chelating agents that can be used in the present invention include alkali metal and/or an alkaline earth metal salts of EDTA (Ethylene Diamine Tetraacetic Acid), for example sodium, potassium, magnesium salts of EDTA, and mixtures of one or more of different salts, preferably, Mg, K, 2K, 2Na, and/or 4Na salts of EDTA. For example, the chelating agent can be (2Na+Mg) salt of EDTA. The other chelating agent known in the art can also be used in the present invention.

The amount of the EDTA salt is at least 0.5% by weight, preferably above 1 % by weight, with respect to the total solids of the lignosulfonates. Generally, 0.5% by weight of the EDTA salt is enough to stabilize the color reduction effect, while the amount of the EDTA salt can be increased according to the practical need. The amount of the EDTA salt may be no more than 5% by weight. Preferably, the amount of the EDTA salt is below 3% by weight, more preferably, below 2% by weight. . The present invention uses relative few materials to provide good stability.

The present method is preferably carried out by adding a reducing agent followed by adding a chelating agent. And, the present method may be carried out at room temperature and atmospheric pressure. The appearance of the calcium lignosulfonate solutions with a concentration of 1 % by weight in water at different pH is shown in figure 1. The as-is solution of calcium lignosulfonate has a pH of about 6. The pH of other solutions is adjusted by the addition of H2SO4 solution. It is quite obvious that the color of the solution decreases dramatically with the decrease of pH in the range from 6 to 4, and such phenomenon is found to be reversible. Hence, this color-pH de- pendency is of no practical usefulness in many practical cases due to the reverse effect and the limitation in the pH range.

Adding a reducing agent (sodium hyposulfite) to the solution at pH 6 with the darkest color, the color reduction effect of this reduction treatment was proved in figure 2. It is quite obvious that under the same lignosulfonates concentration, the color can be lightened to certain level with proper amount of reducing agent.

However, the stability of the color reduction effect using such method is quite low, it turns back to the original appearance in a quite short time (less than one day as shown in figure 3a).

Figure 3b shows the appearance of the sample treated with a reducing agent and a chelating agent together and stored under room temperature for more than 7 days. The color stays at a desired light level. Normally, for the long term storage and long distance transportation, powder phase lignosulfonates are prepared by the spray-drying process directly from the original black liquor. For the color adjustment, the black liquors are treated with the above mentioned reducing agent and chelating agent directly at room temperature with normal stir till homogeneous, and then followed by a spray drying process. The resulted powders are dispersed in water again with the concentration of 1 % to check the appearance. The results still show the light color which indicate that the current treatment can stand the heat and pressure conditions during spray drying.

The powders are stored under 50°C for more than 7 days to evaluate the storage stability there- of. The aqueous solution prepared from the powders which have been subjected to the above heat treatment still shows desired light color, proving again the efficiency, feasibility and the flexibility of the disclosed method in the color reduction.

The present method is a simple method to reduce the color of lignosulfonates. The color reduc- tion effect is achieved by the use of a reducing agent in combination with a chelating agent. The method can be easily embedded into the current normal lignosulfonates production procedure without significant impact to the production cost.

To further prove the effects of the present invention, a number of tests were carried out as fol- lows.

Tests

Test 1

As shown in figure 4, the calcium lignosulfonate aqueous solution (shown as Original solution") with a concentration of 1 % by weight, with the pH at about 6, was a black liquor.

Adding 3% by weight of sodium hydrosulfite with respect to the total solids of the calcium ligno- sulfonate into the aqueous solution (shown as "3wt% Sodium hydrosulfite") at room temperature, the color of the solution was dramatically reduced. Storing the solution at room temperature overnight (shown as "store overnight"), it turned back to the original appearance.

Adding 3% by weight of sodium hydrosulfite with respect to the total solids of the calcium ligno- sulfonate and 1 % by weight of EDTA-4Na with respect to the total solids of the calcium lignosulfonate, into the aqueous solution (shown as "3wt% Sodium hydrosulfite with 1 % EDTA"), the color of the solution was dramatically reduced. Storing the solution at room temperature overnight (shown as "store overnight with 1 % EDTA"), the color was kept substantially unchanged. Further storing the solution at 50°C for 7 days (shown as "store in 50°C 7 days with 1 % EDTA"), the color turned a little darker, but still at a desired light level.

It can be concluded that sodium hydrosulfite has bleaching effect on lignosulfonates, and EDTA salts make the system sufficiently stable to industry requirement. Test 2

As shown in figure 5, the calcium lignosulfonate aqueous solution (shown as Original solution") with a concentration of 1 % by weight, with the pH at about 5.4, was a black liquor. Adding 0.1 mol/L NaOH standard solution to make the pH of the calcium lignosulfonate aqueous solution to be 6 and 8 respectively, and adding 10% by weight of sodium hydrosulfite with respect to the total solids of the calcium lignosulfonate into the aqueous solutions at room temperature, the colors of the solutions were dramatically reduced. Storing the solutions at room temperature overnight (shown as "store overnight"), the solution with a pH of 8 turned darker than the solution with a pH of 6.

Adding 10% by weight of sodium hydrosulfite and 1 % by weight of EDTA-4Na, with respect to the total solids of the calcium lignosulfonate, instead of adding only sodium hydrosulfite, into the aqueous solutions, the color of the solutions were dramatically reduced. Storing the solutions at room temperature overnight, the colors of the solutions were kept substantially unchanged at both pH 6 and pH 8.

It can be concluded that lignosulfonate solutions in high pH are darker and turn back to dark color faster after reduction; and EDTA salts could slow down the darken process, even in high pH solutions.

Test 3 As shown in figure 6, adding 3% by weight of sodium hydrosulfite and 1 % by weight of an EDTA salt, with respect to the total solids of the calcium lignosulfonate, into the calcium lignosulfonate aqueous solution with a concentration of 1 % by weight and a pH of 6 at room temperature, and storing the solutions at room temperature overnight. Different EDTA salts were added respectively, which are EDTA-4Na, EDTA-2Na, EDTA-2K, EDTA-(2Na+Mg).

The colors of all the lignosulfonate solutions kept at a desired light level. It can be concluded that different EDTA salts do not impact the stabilization effect. Test 4

As shown in figure 7, adding 3% by weight of sodium hydrosulfite with respect to the total solids of the calcium lignosulfonate and EDTA-2Na into the calcium lignosulfonate aqueous solution with a concentration of 1 % by weight and a pH of 6, and storing at room temperature overnight. Different amounts of EDTA salts were added respectively, which are 0.5wt%, 1 wt%, 5wt%, and 10wt%, with respect to the total solids of the calcium lignosulfonate.

The color of all the lignosulfonate solutions kept at a desired light level.

It can be concluded that 0.5% by weight of the EDTA salt is enough to stabilize the color reduction effect, the stabilization effect does not rise significantly with EDTA contents. Test 5

As shown in figure 8, adding sodium hydrosulfite into the calcium lignosulfonate aqueous solution with a concentration of 1 % by weight and a pH of 5.4 at room temperature. Different amounts of sodium hydrosulfite were added respectively, which are 0.5wt%, 1 wt%, and 2wt%, with respect to the total solids of the calcium lignosulfonate.

The colors of the calcium lignosulfonate aqueous solutions are gradually lighter with rising of content of sodium hydrosulfite. Nevertheless, the calcium lignosulfonate aqueous solution with addition of 0.5% by weight of sodium hydrosulfite has already had a desired light color, so 0.5% by weight of sodium hydrosulfite has already achieved a desired bleaching effect.

It can be concluded that the bleaching effect turns more obviously with rising of content of sodium hydrosulfite. And, 0.5% by weight of sodium hydrosulfite is enough to achieve a desired bleaching effect.

Test 6

As shown in figure 9, the original calcium lignosulfonate solution with a concentration of 1 % by weight and a pH of 5.4 was treated with 1 % by weight of sodium hydrosulfite and 1 % by weight of EDTA-4Na, with respect to the total solids of the calcium lignosulfonate at room temperature with normal stir till homogeneous. The resulted solution was followed by a spray drying process at a temperature of 105-130°C. One part of the resulted powders was dissolved in water again to obtain a solution with a concentration of 1 % by weight and a pH of 5.4, the solution was then stored at 50°C for 7 days, the color reduction effect kept at a desired level. Another part of the resulted powders was stored at 50°C for 7 days, then dissolved in water to obtain a solution with a concentration of 1 % by weight and a pH of 5.4 to check the appearance of the solution, the color reduction effect also kept at a desired level. It can be concluded that spraying dry process does not reduce bleaching effect.

Claims

A method for reducing the color of lignosulfonates, comprising mixing a reducing agent and a chelating agent with the lignosulfonates.

The method for reducing the color of lignosulfonates according to claim 1 , which is carried out by adding a reducing agent followed by adding a chelating agent into the lignosulfonates.

The method for reducing the color of lignosulfonates according to claim 1 or 2, which is carried out at room temperature and atmospheric pressure.

The method for reducing the color of lignosulfonates according to any of claims 1 -3, wherein the chelating agent is an EDTA salt.

The method for reducing the color of lignosulfonates according to claim 4, wherein the EDTA salt comprises an alkali metal and/or an alkaline earth metal salt of EDTA.

The method for reducing the color of lignosulfonates according to claim 5, wherein the EDTA salt comprises Mg, K, 2K, 2Na, and/or 4Na salts of EDTA.

The method for reducing the color of lignosulfonates according to any of claims 1 -6, wherein the amount of the chelating agent is at least 0.5% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to claim 7, wherein the amount of the chelating agent is from 0.5% by weight to 10% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to claim 7, wherein the amount of the chelating agent is from 0.5% by weight to 5% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to claim 7, wherein the amount of the chelating agent is from 0.5% by weight to 2% by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to claim 7, wherein the amount of the chelating agent is from 0.5% by weight to 1 % by weight, with respect to the total solids of the lignosulfonates.

The method for reducing the color of lignosulfonates according to any of claims 1 -1 1 , wherein the reducing agent is a hydrosulfite.

13. The method for reducing the color of lignosulfonates according to claim 12, wherein the reducing agent is sodium hydrosulfite.

14. The method for reducing the color of lignosulfonates according to any of claims 1 -12, wherein the amount of the reducing agent is above 0.5% by weight, with respect to the total solids of the lignosulfonates.

15. The method for reducing the color of lignosulfonates according to claim 14, wherein the amount of the reducing agent is from 0.5% by weight to 10% by weight, with respect to the total solids of the lignosulfonates.

16. The method for reducing the color of lignosulfonates according to claim 15, wherein the amount of the reducing agent is from 0.5% by weight to 5% by weight, with respect to the total solids of the lignosulfonates.

17. The method for reducing the color of lignosulfonates according to claim 16, wherein the amount of the reducing agent is from 0.5% by weight to 3% by weight, with respect to the total solids of the lignosulfonates.

18. The method for reducing the color of lignosulfonates according to claim 17, wherein the amount of the reducing agent is from 0.5% by weight to 2% by weight, with respect to the total solids of the lignosulfonates.